This invention relates to electronic circuits of the general type used to process audio signals in order to effect a reduction in noise produced during recording and included in the signal, while not audibly degrading the portion of the signal intended for listening. While the various circuits disclosed in this application are useful in a wide variety of different types of noise reduction and related systems, one particular use for which it is intended is in connection with AM, FM and television audio recording, transmission and reception.
The circuits according to the invention disclosed in this application strike a scientifically valid and economically feasible balance which will permit early incorporation of noise reduction circuits in television equipment which will provide technically excellent results at reasonable cost.
In general, the invention relates to detectors which are suitable for use in compressors, expandors, compandors, limiters and other types of noise reduction systems. The circuit permits a faster basic attack time than known circuits. The circuit also delivers a higher detector output voltage by avoiding or minimizing output detector source voltage attenuation. In circuits according to the present invention, the discharge diode is placed within a feedback loop of the operational amplifier so that the hold time of the system is unaffected by the detector level.
The more specific disclosures of the present invention which follow should be read and understood in view of information concerning other types of noise reduction systems. For example, FIG. 1 of this application discloses a detector circuit commercialized in 1976 by Robert Orban and first applied in a limiter known as the "FM Optimod". FIG. 2 discloses a circuit of the general type disclosed in U.S. Pat. No. 4,250,471 granted on Feb. 10, 1981. Reference is also made to an extensive discussion contained under the heading "Discussion of the Prior Art" contained in columns 1 through 6 of the '471 patent. One basic difference between the Orban circuit and that disclosed in the '471 patent is that the latter includes a resistive voltage divider R4-R5 to provide a drive level for detector 2(D2) proportional to and less than the input signal. The hold function of the Orban circuit therefore varies greatly as a function of the amplitude of Vin.